Battery type plays an important role in maintain the battery.
Over the years Nicad->NiMH->li-ion.
Nicad; great for High Current, short time but had memory problems. primarily because individuals did not charge/discharge properly. Still used in power tools. Should always discharge to 1 volt per cell -NO LOWER. and charge at 1/10 AmpHr rating for 16 hours. Charging E should be limited to approx 1.35 V. storage - discharged
Replaced by NiMH which drastically reduced memory effect - same charge dischare requirements. Primary usage today, also power tools.
Li-ion. Higher volts per cell. No memory problem. Best for laptops. Storage - should be about 40% charged level.
http://centralhobbies.com/instructional/lithium.html
Concur with jaguarskx on removing battery when under prolonged usage of charger, But NOT because of overcharging. The circuitry should prevent this and longest life is reduced charge/discharge. The reason is more temperature driven
Quote:
For maximum storage life, batteries should be stored with a 40% charge (3.6 V) at 40°F (refrigerator). Perhaps one of the worst locations for a Li-ion battery is in a laptop computer when used daily on a desktop with the charger connected. Laptops typically run warm or even hot, raising the battery temperature, and the charger is maintaining the battery near 100% charge. Both of these conditions shorten battery life, which could be as short as six months to a year. If possible, the user should be instructed to remove the battery and use the ac adapter for powering the laptop when used as a desktop computer. A properly cared for laptop battery can have a service life of two to four years or more. END QUOTE
Ref: http://powerelectronics.com/portable_power_management/battery_charger_ics/proper_care_extends-li-ion-battery-0425/
Also an excellent artical on Li-ion batteries in general.